A professed democratic socialist whose calls for “Medicare for all,” a $15 minimum wage and tuition-free public colleges have become pillars of the party’s left wing, Mr. Sanders joins the race at a time when Republicans are trying to define the Democratic field and its ideas as out of the political mainstream. In Mr. Sanders, who has not joined the Democratic Party, Republicans have an easy target to try to make the face of the opposition.

But Mr. Sanders, 77, starts with stronger support from small-dollar donors and liberal voters than most other candidates. And he is among the best-known Democrats in a crowded field, as well as one of the most outspoken against President Trump, whom he has repeatedly called a “pathological liar” and a “racist.”

“During our 2016 campaign, when we brought forth our progressive agenda, we were told that our ideas were ‘radical’ and ‘extreme,’” Mr. Sanders said on Tuesday in an email to supporters. “Three years have come and gone. And, as result of millions of Americans standing up and fighting back, all of these policies and more are now supported by a majority of Americans.”

This time around, Mr. Sanders, enters the race at a far different electoral moment. Much of his populist agenda has been embraced by other Democrats, at a time when many voters are eager to elevate female and nonwhite standard bearers. He will no longer have the Clinton dynasty as a foil; instead, his competition will include progressives like Senator Elizabeth Warren of Massachusetts, who has broadly supported many of the same economic positions for years.

And he will face far more scrutiny than three years ago, when much of the news media and political class treated him as more of an outlier than as a genuine challenger for the nomination. Already, he has had to quell the unease about his campaign’s treatment of women that has been disclosed in recent weeks and prompted two public apologies. His stumbles on issues of race and identity continue to concern activists who fear he has learned little from his previous White House bid.

Whether Mr. Sanders can break through in a crowded field of diverse candidates, many of whom champion the progressive message he made popular, could go a long way in determining the direction of the Democratic Party in the age of Trump.

“A lot of people still believe that he is the one who can take Trump out,” said Yvette Simpson, chief executive of the political group Democracy for America. The question now, she said, is “how does he distinguish himself in that bigger field?”

Republicans have seized on Mr. Sanders’s entrance, eager to ascribe the socialist label to all Democrats. Soon after his announcement, the Trump re-election campaign issued a statement denouncing “every” Democratic candidate for “embracing his brand of socialism.” The president said pointedly that Mr. Sanders “missed his time.”

In an interview on “CBS This Morning,” Mr. Sanders did not shy away from calling himself a democratic socialist. Mr. Trump, Mr. Sanders said, is “going to say, ‘Bernie Sanders wants the United States to become Venezuela.’”

“Bernie Sanders does not want to have the United States become the horrific economic situation that unfortunately exists in Venezuela right now,” he said. “What Bernie Sanders wants is to learn from countries around the world why other countries are doing a better job of dealing with income and wealth inequality than we are.”

Mr. Sanders will start with several advantages, including the foundation of a 50-state organization; a massive lead among low-dollar donors that is roughly equivalent to the donor base of all the other Democratic hopefuls combined; and a cache of fervent, unwavering supporters who spent the day exulting in his decision to run. A coveted speaker, he can still electrify crowds in a way few politicians can. He enjoys wide name recognition, and several early polls had him running second behind former Vice President Joseph R. Biden Jr.

The strength of that donor list was quickly apparent: In the 12 hours after Mr. Sanders formally joined the race, his campaign said it had pulled in more than $4 million.

And while rising stars like Alexandria Ocasio-Cortez and Ayanna Pressley have siphoned some of his authority over the party’s progressive wing, Mr. Sanders still claims to have spawned a “political revolution” that, true revolution or not, has ignited a generation of young, socialist-leaning voters and reshaped the Democratic Party.

He is also partly responsible for the party’s decision last year to overhaul its presidential nomination process, including sharply reducing the influence of superdelegates and increasing the transparency around debates — factors he felt greatly favored Hillary Clinton in 2016.

Asked what would be different in 2020, Mr. Sanders replied bluntly: “We’re going to win.”

“Bottom line,” he said, “it is absolutely imperative that Donald Trump be defeated.” Though he had harsh words for the president, he said he was fond of the five other senators who wererunning for the Democratic nomination. “They are in some cases my friends,” he said.

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Senator Bernie Sanders is embarking on a second run for president. This time the field will be bigger, more diverse and filled with candidates who have adopted his progressive populist mantle.CreditCreditSam Hodgson for The New York Times

Mr. Sanders did not immediately announce where he would campaign first. Faiz Shakir, the national political director at the A.C.L.U. and a former adviser to Senator Harry Reid of Nevada, the former majority leader, will serve as campaign manager.

With his booming voice and familiar wide-armed grip at the lectern, Mr. Sanders has long positioned himself as a champion of the working class and a passionate opponent of Wall Street and the moneyed elite. His remarks often include diatribes against the millionaires and billionaires — one of his most common refrains is that the “three wealthiest people in America own more wealth than the bottom 50 percent” — as well as denunciations of “super PACs” and the influence of big money on politics. In particular, he has sharply criticized Amazon and Walmart over their wages and treatment of workers.

In his email to supporters, as well as a campaign announcement video, Mr. Sanders laid out a litany of policy issues, familiar to anyone who has followed him: universal health care, tuition-free public college, women’s reproductive rights, lower prescription drug prices, criminal justice reform.

“Our campaign is about taking on the powerful special interests that dominate our economic and political life,” he said.

While some presidential candidates have avoided direct broadsides against President Trump, Mr. Sanders, ever combative, addressed his potential opponent head on.

“We are running against a president who is a pathological liar, a fraud, a racist, a sexist, a xenophobe and someone who is undermining American democracy as he leads us in an authoritarian direction,” he said.

Born in Brooklyn, with an accent to match, Mr. Sanders ran unsuccessfully in the 1970s for governor and United States senator in Vermont before being elected mayor of Burlington in 1981. For 16 years, he served as the only congressman in the state before he was elected to the Senate in 2006.

Mr. Sanders has been a modest legislator and something of a lone wolf in Washington, promoting largely the same legislative agenda since his early days as a mayor. He voted against the Iraq War and, in 2008, was one of roughly two dozen senators to vote against the $700 billion bailout of big banks.

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Senator Bernie Sanders of Vermont marched with others from a prayer service at Zion Baptist Church in Columbia, S.C., to the state house last month during an event on Martin Luther King Jr. Day.CreditTravis Dove for The New York Times

While often viewed as a pesky left-wing gadfly, he is also known to reach across the aisle, working on legislation with Senator Charles E. Grassley of Iowa and Senator John McCain of Arizona, both Republicans. He has rationalized voting for the 1994 crime bill, now heavily criticized for some of its draconian provisions, by saying he had favored progressive parts of the bill, including the Violence Against Women Act, while strongly opposing measures that would lead to mass incarceration.

Mr. Sanders is the longest-serving independent in congressional history, a point of pride for him but one of consternation and annoyance for some Democrats who are quick to suggest he does not have the party’s interests at heart. Some Democrats blame him for Mrs. Clinton’s loss in 2016, saying his anti-establishment rhetoric during his campaign inflamed divisions in the party that proved insurmountable.

One 2016 issue that has resurfaced is his record on gun control, Democratic strategists have said, given the intensity of the debate around gun violence following recent mass shootings. In 2005, Mr. Sanders voted for a law that granted immunity to gun manufacturers and dealers from most liability lawsuits. Mr. Sanders has also been criticized for support he received from the N.R.A. when he was running for Congress in 1990, in part because he vowed not to support a bill that mandated a waiting period for handgun sales.

Though his message is well worn, Mr. Sanders has indicated that he is trying to remedy weaknesses from his first presidential campaign, including his lack of support from black voters. In recent months, he has made a series of trips to the South, where in 2016 he drew less than 20 percent of the black vote. During the Martin Luther King Jr. holiday, he made a swing through South Carolina — where black voters made up about 60 percent of the Democratic primary vote in 2016 — that included addressing supporters and students and speaking with lawmakers.

He has also tried to shore up his foreign policy credentials, becoming a critic of the United States support of Saudi Arabia’s war in Yemen. Late last year, the Senate passed a resolution, which Mr. Sanders helped introduce, to end American military assistance for the kingdom’s war there.

But he also continues to draw ire from critics who say that the way he talks about race and identity is out of step with the calls for diversity and change within the party.

Almost immediately after making his announcement, Mr. Sanders was attacked for his response to a question on Vermont Public Radio about whether he thought he best represented the current Democratic Party.

“We have got to look at candidates, you know, not by the color of their skin, not by their sexual orientation or their gender and not by their age,” Mr. Sanders said. “I think we have got to try to move us toward a nondiscriminatory society which looks at people based on their abilities, based on what they stand for.”

Whether Mr. Sanders can secure the Democratic nomination may depend on his ability to change his timeworn message — and adapt to the demands and yearnings of a party that he helped shape.

“Before 2016, nobody was really talking about universal health care and universal education as much as he was; he really brought that into the mainstream for a lot of people,” said Dave Degner, the chair of the Tama County Democratic Party in Iowa. “If he comes out saying the same things that he did before, it’s not new anymore.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Once an Outlier, Sanders Enters A Packed Race. Order Reprints | Today’s Paper | Subscribe